Multimodal polyethylene pipe

ABSTRACT

The present invention relates to a reactor system for a multimodal polyethylene composition comprising: (a) first reactor (b) hydrogen removal unit arranged between the first reactor and a second reactor comprising at least one vessel connected with a depressurization equipment, preferably selected from vacuum pump, compressor, blower, ejector or a combination thereof, the depressurization equipment allowing to adjust an operating pressure to a pressure in a range of 100-200 kPa (abs); (c) the second reactor; and (d) a third reactor and use thereof as a pipe.

The present invention relates to a reactor system for a multimodal polyethylene polymerization process, a process for producing a multimodal polyethylene composition using said reactor system, a multimodal polyethylene composition obtainable this way as well as to a pipe formed by using said multimodal polyethylene composition.

The demand of polyethylene resins is increasingly being used in a variety of applications. As required high performance of polyethylene for a relatively new plastic, a polymerization process technology has been developed to support new polymeric material production. In order for balancing processability and physical properties of ethylene copolymers, the development in multimodal polymerization process has been investigated.

In the prior art, multimodal polyethylene polymerization is employed to produce polymers having different molecular weights by creating each resin fraction in separated reactors. A low molecular weight fraction is produced in a reactor using an excess of hydrogen to control the molecular weight of the polymer suitable for providing good processability of the final polymer.

A high molecular weight fraction which has an influence on the physical properties and is produced under polymerization conditions with low hydrogen concentration. It is well known in the art that low molecular weight polymer is preferably produced in a first reactor. To obtain a multimodal polymer with good physical properties, all hydrogen from the first reactor should be removed before the polymerized slurry polymer is passed to a second reactor in which the production of high molecular weight polymer takes place.

US2010/0092709 A1 describes a process for preparing bimodal polyethylene copolymers. The polymerization in a second reactor is operated at a high temperature with a low comonomer-to-ethylene-ratio and low hydrogen-to-ethylene-ratio to obtain resins having improved stress crack resistance and melt strength.

U.S. Pat. No. 6,716,936 B1 describes a process for producing bimodal polyethylene copolymers. A second reactor is operated under hydrogen depleted polyethylene polymerization by directing a polyethylene slurry stream from a first reactor to a hydrogen removal system. Polymerization in both the first and the second reactors is operated at the bubble point by using propane or is butane as a light solvent. The process is suitable for the production of a bimodal polyethylene for highly homogeneous high molecular weight resins.

U.S. Pat. No. 6,291,601 B1 describes a process for producing a bimodal copolymer with relatively high molecular weight polyethylene. A hydrogenation catalyst is introduced into a second reactor to consume residual hydrogen gas from first reactor by converting hydrogen into ethane leading to a low hydrogen concentration in the second reactor. Using this technique, the cost of raw material consumption of both hydrogen and ethylene are increased due to converting of unreacted gases.

US 2003/0191251 A1 discloses a process for removing residual hydrogen from a polymer slurry by using two flash drums placed between cascade reactors which use light solvent as a diluent.

The addition of make-up solvent to the first flash drum outlet is required to prevent a slurry transfer pump blocking. Furthermore, warm make-up solvent is necessary before transferring slurry into the next flash drum.

EP 1 655 334 A1 discloses the multimodal production of an ethylene polymer which is produced in a multistage process with a MgCl₂-based Ziegler-Natta catalyst. The polymerization stages are performed in the following order to achieve firstly a ultra high molecular weight polymer, followed by achieving a low molecular weight polymer, and finally achieving high molecular weight polymer in the last step. The polymerization catalyst is charged to a prepolymerization step to make an ultrahigh molecular weight fraction.

WO 2013/144328 describes a composition of multimodal high density polyethylene which is produced using a Ziegler-Natta catalyst for use in molding applications. A small fraction of ultra-high polyethylene of less than 15% by weight is produced in a third reactor.

US 2009/0105422 A1 describes a process for producing a multimodal polyethylene. The polymerization is carried out in three cascade reactors, wherein the molecular weight of the polymer in each reactor is controlled by the presence of hydrogen. The concentration of the hydrogen in each reactor is reduced subsequently by providing the highest hydrogen concentration in the first reactor and the lowest hydrogen concentration in the third reactor.

WO 2013/113797 describes a process for polyethylene preparation comprising three main subsequent steps of polymerized ethylene and at least one other α-olefin to get the polyethylene with, respectively, a lower molecular weight ethylene polymer, a first higher molecular weight ethylene polymer and a second higher molecular weight ethylene polymer in accordance with the sequence of a first reactor, a second reactor and a third reactor.

Even though many processes for preparing multimodal polyethylene are known and have been described, there is still a need for developing new processes for multimodal polymerization, particularly for further improving the mechanical properties of polyethylene compositions.

Therefore, it is the object of the present invention to provide a reactor system and a process for preparing multimodal polyethylenes overcoming drawbacks of the prior art, in particular to enhance the performance of a hydrogen removal unit comprised in such a reactor.

It is a further object to provide a multimodal polyethylene composition overcoming drawbacks of the prior art, in particular having improved mechanical properties, such as Charpy index.

It is well known in the art that polyethylene pipes are widely used in various industrial applications, in which the fluids can be pressurized or non-pressurized. According to ISO 9080, a new pressure designation for PE would be made to achieve PE125 requirements. This means that the pipes having a Minimum Required Strength (MRS) of 12.5 MPa for 50 years at 20° C. without fracturing. In order to improve hydrostatic pressure resistance from the PEI00 standard to PE125 performance, either an increased intercept or a decreased slope of the ductility is required.

The incorporation of an ultrahigh molecular weight polyethylene (UHMWPE) polymer into a polyethylene composition as a copolymer is also known and disclosed, for example, WO 2007/042216, WO 96/18677 and WO 2006/092378.

EP 1201713 A1 disclosed the blending of a high molecular weight polyethylene fraction containing a linear low density polyethylene with a density of 0.928 g/cm³ and a low molecular weight polyethylene fraction comprising a high density polyethylene with a density of at least 0.969 g/cm³.

The blending of 10% UHMW polyethylene into 90% bimodal HDPE using a co-rotating twin screw extruder via extrusion was reported in EP 2743 305 A1. However, the extrusion for at least 3-4 times were required to dispersed the UHMW and HDPE matrix. This is also the main problem for HDPE containing UHMWPE in bimodal polymerization reported in EP 1417260 B1. For these reasons, the content of UHMW polyethylene should be concerned to limited loadings.

It is therefore a further object of the present invention to provide a pipe formed of a multimodal polyethylene composition overcoming drawbacks of the prior art, in particular having improved mechanical properties and allowing the pipe to withstand wall stress of 12.5 MPa at 20° C. without fracturing. It is desired to produce an UHMWPE into a multimodal high density polyethylene matrix at commercially relevant loadings without multiple extrusion. Furthermore, the advantage of UHMW incorporation in multimodal PE pipe is the sagging resistance to maintain uniform dimensions around the pipe wall from gravity flow of polymer melt during pipe extrusion.

These objects are achieved in accordance with the invention according to the subject-matter of the independent claims. Preferred embodiments result from the sub-claims.

The object is first of all achieved by a reactor system for a multimodal polyethylene polymerization process, comprising:

-   -   (a) a first reactor;     -   (b) a hydrogen removal unit arranged between the first reactor         and a second reactor comprising at least one vessel connected         with a depressurization equipment, preferably selected from         vacuum pump, compressor, blower, ejector or a combination         thereof, the depressurization equipment allowing to adjust an         operating pressure to a pressure in a range of 100-200 kPa         (abs);     -   (d) the second reactor; and     -   (e) a third reactor.

Preferably, the depressurization equipment allows to adjust the operating pressure in the hydrogen removal unit to a pressure in the range of 103-145 kPa (abs), preferably 104-130 kPa (abs), most preferably 105 to 115 kPa (abs) Preferably, the hydrogen removal unit further contains a stripping column for the separation of hydrogen and a liquid diluent.

The object is further achieved by a process for producing a multimodal polyethylene composition in an inventive reactor system, comprising (in this sequence);

-   -   (a) polymerizing ethylene in an inert hydrocarbon medium in the         first reactor in the presence of a catalyst system, selected         from Ziegler-Natta catalyst or metallocene, and hydrogen in an         amount of 0.1-95% by mol with respect to the total gas present         in the vapor phase in the first reactor to obtain a low         molecular weight polyethylene having a weight average molecular         weight (Mw) of 20,000 to 90,000 g/mol or medium molecular weight         polyethylene having a weight average molecular weight (Mw) of         more than 90,000 to 150,000 g/mol wherein the low molecular         weight polyethylene, respectively the medium molecular weight         polyethylene, has a density at least 0.965 g/cm3, and the low         molecular weight polyethylene has MI2 in the range from 10 to         1,000 g/10 min and the medium molecular weight polyethylene has         MI2 in the range from 0.1 to 10 g/10 min;     -   (b) removing in the hydrogen removal unit 98.0 to 99.8% by         weight of the hydrogen comprised in a slurry mixture obtained         from the first reactor at a pressure in the range of 103-145 kPa         (abs) and transferring the obtained residual mixture to the         second reactor,     -   (c) polymerizing ethylene and optionally C₄ to C₁₂ α-olefin         comonomer in the second reactor in the presence of a catalyst         system, selected from Ziegler-Natta catalyst or metallocene, and         in the presence of hydrogen in an amount obtained in step (b) to         obtain a first high molecular weight polyethylene having a         weight average molecular weight (Mw) of more than 150,000 to         1,000,000 g/mol or a first ultra high molecular weight         polyethylene in the form of a homopolymer or a copolymer having         a weight average molecular weight (Mw) of more than 1,000,000 to         5,000,000 g/mol and transferring a resultant mixture to the         third reactor, and     -   (d) polymerizing ethylene, and optionally C₄ to C₁₂ α-olefin         comonomer in the third reactor in the presence of a catalyst         system, selected from Ziegler-Natta catalyst or metallocene, and         hydrogen, wherein the amount of hydrogen in the third reactor is         in a range of 0.1-70% by mol, preferably 0.1-60% by mol with         respect to the total gas present in the vapor phase in the third         reactor or optionally substantial absence of hydrogen to obtain         a second high molecular weight polyethylene having a weight         average molecular weight (Mw) of more than 150,000 to 1,000,000         g/mol or a second ultra high molecular weight polyethylene         homopolymer or copolymer having a weight average molecular         weight (Mw) of more than 1,000,000 to 5,000,000 g/mol.

“Substantially absence” in this regard means that hydrogen is only comprised in the third reactor in an amount which cannot be avoided by technical means.

The slurry mixture obtained from the first reactor and subjected to the step of removing hydrogen in the hydrogen removal unit contains all of the solid and liquid constituents obtained in the first reactor, in particular the low molecular weight polyethylene or the medium molecular weight polyethylene. Furthermore, the slurry mixture obtained from the first reactor is saturated with hydrogen regardless the amount of hydrogen used in the first reactor.

Preferably, the removing is removing of 98.0 to 99.8% by weight of the hydrogen, and more preferable 98.0 to 99.5% by weight, most preferred 98.0 to 99.1% by weight.

Preferably, the operation pressure of the hydrogen removal unit is in the range of 103-145 kPa(abs) and more preferably 104-130 kPa (abs), most preferred 105 to 115 kPa (abs).

Preferably, step (a) results in the low molecular weight polyethylene or medium molecular weight polyethylene, step (c) results in high molecular weight polyethylene or the ultra high molecular weight polyethylene, and step (d) results in high molecular weight polyethylene or the ultra high molecular weight polyethylene.

The weight average molecular weight (Mw) of the low molecular weight polyethylene, the medium molecular weight polyethylene, the high molecular weight polyethylene and the ultra high molecular weight polyethylene described herein are in the range of 20,000-90,000 g/mol (low), more than 90,000-150,000 g/mol (medium), more than 150,000-1,000,000 g/mol (high) and more than 1,000,000-5,000,000 g/mol (ultra high) respectively.

Moreover, the object is achieved by a multimodal polyethylene composition obtainable by the inventive process, comprising;

(A) 30 to 65 parts by weight, preferably 40 to 65 parts by weight, more preferably 50 to 60 parts by weight of the low molecular weight polyethylene or the medium molecular weight polyethylene; (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight of the first high molecular weight polyethylene or the first ultra high molecular weight polyethylene; and (C) 25 to 40 parts by weight, preferably 30 to 38 party by weight, of the second high molecular weight polyethylene or the second ultra high molecular weight polyethylene, wherein the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the multimodal polyethylene composition is from 10 to 60, preferably 10 to 40, preferably 20 to 30, wherein Mw and Mn are respectively determined by gel permeation chromatography.

The object is further achieved by a multimodal polyethylene composition comprising

(A) 40 to 65 parts by weight, preferably 50 to 60 parts by weight of a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g/mol or a medium molecular weight polyethylene having a weight average molecular weight (Mw) of more than 90,000 to 150,000 g/mol; (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight of a first high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a first ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol; and (C) 25 to 40 parts by weight, preferably 30 to 38 party by weight of a second high molecular weight polyethylene copolymer having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a second ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol, wherein the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the multimodal polyethylene composition is from 10 to 60, preferably 20 to 30, wherein Mw and Mn are respectively determined by gel permeation chromatography and has a crystallinity of at least 57%, preferably of at least 60% determined by differential scanning calorimetry.

Preferably, the multimodal polyethylene composition has a crystallinity of at least 60%, preferably from 57 to 70%, preferably 60 to 70% determined by differential scanning calorimetry.

In a preferred embodiment, the multimodal polyethylene composition has a weight average molecular weight from 80,000 to 1,300,000 g/mol, preferably 200,000 to 400,000 g/mol, preferably 250,000 to 350,000 g/mol, measured by Gel Permeation Chromatography.

Furthermore, it is preferred, that the multimodal polyethylene composition has a number average molecular weight from 5,000 to 30,000 g/mol, preferably 8,000 to 20,000 g/mol, more preferably 10,000 to 15,000 g/mol measured by Gel Permeation Chromatography.

Preferably, the multimodal polyethylene composition has a Z average molecular weight from 800,000 to 6,000,000 g/mol, preferably 800,000 to 3,000,000 g/mol, preferably 1,000,000 to 2,500,000 g/mol, measured by Gel Permeation Chromatography.

Preferably, the multimodal polyethylene composition has a density from 0.940 to 0.965 g/cm³, preferably 0.945 to 0.965 g/cm³ preferably from 0.958 to 0.963 g/cm³ according to ISO 1183 and/or a melt flow index MI₅ from 0.1 to 60 g/10 min, preferably 0.1 to 0.4 g/10 min, preferably from 0.15 to 0.3 g/10 min,

It is further preferred that the multimodal polyethylene composition has a melt flow index MI₅ from 0.1 to 0.5 g/10 min, preferably 0.190 to 0.4 g/10 min, preferably 0.195 to 0.275 g/10 min.

More preferred, the multimodal polyethylene composition has an average lamella repeating period comprising a crystalline portion and an amorphous portion wherein the crystalline portion has an average thickness of at least 7.2 nm and the amorphous portion has an average thickness of at least 19 nm.

In a preferred embodiment, the crystalline portion has an average thickness from 7.2 nm to 9.0 nm, preferably 7.2 nm to 8.5 nm, preferably 7.2 nm to 8.3 nm, preferably 7.23 to 8.22 nm; and/or the amorphous portion has an average thickness from 19 nm to 30 nm, preferably 19 nm to 25 nm, preferably 19 nm to 23 nm, preferably 19 nm to 22 nm, preferably 19.55 nm to 21.38 nm.

Furthermore, it is preferred that an alpha temperature of multimodal polyethylene composition is at least 60° C., preferably of at least 63° C., more preferably of 63 to 70° C., most preferred from 63 to 65° C.

Preferably, the alpha temperature is from 63.46 to 64.46° C.

The object is further achieved by a layer consisting of the inventive multimodal polyethylene composition, wherein the layer comprises at least one crystalline portion and at least one amorphous portion, wherein the thickness of the crystalline portion perpendicular to the surface of the layer is at least 7.2 nm and the thickness of the amorphous portion perpendicular to the surface of the layer is at least 19 nm.

Most preferably the layer has an alpha temperature of at least 60° C., preferably of at least 63° C., more preferably of 63 to 70° C., most preferred from 63 to 65° C.

The object is further achieved by a pipe, wherein a wall of the pipe is an inventive layer.

Finally the object is achieved by the use of the inventive pipe for applications under a pressure of at least 12.5 MPa.

In preferred embodiments of the inventive reactor system, the inventive process and the inventive multimodal polyethylene composition “comprising” is “consisting of”.

In preferred embodiments “parts by weight” is “percent by weight”.

The above embodiments mentioned to be preferred resulted in even more improved mechanical properties of the obtained multimodal polyethylene composition. Best results were achieved by combining two or more of the above preferred embodiments. Likewise, the embodiments mentioned above to be more or most preferred resulted in the best improvement of mechanical properties.

Multimodal polyethylene polymers are well known in the art. A multimodal polyethylene system typically consists of a high molecular weight (HMW) component and a low molecular weight (LMW) component. The HMW component contributes good mechanical properties to the system, whilst the LMW component provides good processability. Improved mechanical properties can be achieved by increasing the molecular weight of the HMW component. The combination of a specific high density multimodal polyethylene polymer with a particular copolymer ultrahigh molecular weight polyethylene component and copolymer high molecular weight polyethylene part can result in a polymer which provides the balance properties that meet the requirements of PE125 and good slow crack growth (SCG) with good sagging resistance.

Moreover, these components can be homogeneously blended with the very high molecular weights incorporation.

The invention concerns a reactor system for multimodal polyethylene polymerization. The system comprises a first reactor, a second reactor, a third reactor and a hydrogen removal unit placed between the first reactor and the second reactor.

The hydrogen depleted polyethylene from the first reactor affects the polymerization of high molecular weight in the subsequent reactors. In particular, high molecular weight leads to improved mechanical properties of polyethylene that is the advantage for various product application includes injection molding, blow molding and extrusion. The catalyst for producing the multimodal polyethylene resin of this invention is selected from a Ziegler-Natta catalyst, a single site catalyst including metallocene-bases catalyst and non-metallocene-bases catalyst or chromium based might be used, preferably conventional Ziegler-Natta catalyst or single site catalyst. The catalyst is typically used together with cocatalysts which are well known in the art.

Inert hydrocarbon is preferably aliphatic hydrocarbon including hexane, isohexane, heptane, isobutane. Preferably, hexane (most preferred n-hexane) is used. Coordination catalyst, ethylene, hydrogen and optionally α-olefin comonomer are polymerized in the first reactor. The entire product obtained from the first reactor is then transferred to the hydrogen removal unit to remove 98.0 to 99.8% by weight of hydrogen, unreacted gas and some volatiles before being fed to the second reactor to continue the polymerization. The polyethylene obtained from the second reactor is a bimodal polyethylene which is the combination of the product obtained from the first reactor and that of the second reactor. This bimodal polyethylene is then fed to the third reactor to continue the polymerization. The final multimodal (trimodal) polyethylene obtained from the third reactor is the mixture of the polymers from the first, the second and the third reactor.

The polymerization in the first, the second and the third reactor is conducted under different process conditions. As a result, the polyethylene obtained in each reactor has a different molecular weight. Preferably, low molecular weight polyethylene or medium molecular weight polyethylene is produced in the first reactor, while high molecular weight polyethylene or ultra high molecular weight polyethylene is produced in the second and third reactor respectively.

The term first reactor refers to the stage where the low molecular weight polyethylene (LMW) or the medium molecular weight polyethylene (MMW) is produced. The term second reactor refers to the stage where the first high or ultra high molecular weight polyethylene (HMW1) is produced. The term third reactor refers to the stage where the second high molecular weight polyethylene or ultra high molecular weight (HMW2) is produced.

The term LMW refers to the low molecular weight polyethylene polymer polymerized in the first reactor having a weight average molecular weight (Mw) of 20,000-90,000 g/mol.

The term MMW refers to the medium molecular weight polyethylene polymer polymerized in the first reactor having a weight average molecular weight (Mw) of more than 90,000-150,000 g/mol.

The term HMW1 refers to the high or ultra high molecular weight polyethylene polymer polymerized in the second reactor having a weight average molecular weight (Mw) of more than 150,000 to 5,000,000 g/mol.

The term HMW2 refers to the high or ultra high molecular weight polyethylene polymer polymerized in the third reactor having the weight average molecular weight (Mw) of more than 150,000 to 5,000,000 g/mol.

The LMW or MMW is produced in the first reactor in the absence of comonomer in order to obtain a homopolymer.

To obtain the improved polyethylene properties of this invention, ethylene is polymerized in the first reactor in the absence of comonomer in order to obtain high density LMW polyethylene or MMW polyethylene having density ≥0.965 g/cm³ and MI₂ in the range of 10-1000 g/110 min for LMW and 0.1-10 g/10 min for MMW. In order to obtain the target density and MI in the first reactor, the polymerization conditions are controlled and adjusted. The temperature in the first reactor ranges from 70-90° C., preferably 80-85° C. Hydrogen is fed to the first reactor so as to control the molecular weight of the polyethylene. The molar ratio of hydrogen to ethylene in the vapor phase can be varied depending up on the target MI. However, the preferred molar ratio ranges from 0.01-8.0, more preferably 0.01-6.0. The first reactor is operated at pressure between 250 and 900 kPa, preferably 400-850 kPa. An amount of hydrogen present in the vapor phase of the first reactor is in the range of 0.1-95% by mole, preferably 0.1-90% by mol.

Before being fed to the second reactor, the slurry obtained from the first reactor containing LMW or MMW polyethylene preferably in hexane is transferred to a hydrogen removal unit which may have a flash drum connected with depressurization equipment preferably including one or the combination of vacuum pump, compressor, blower and ejector where the pressure in the flash drum is reduced so that volatile, unreacted gas, and hydrogen are removed from the slurry stream. The operating pressure of the hydrogen removal unit typically ranges from 103-145 kPa (abs), preferably 104-130 kPa (abs) in which 98.0 to 99.8% by weight of hydrogen can be removed, preferably 98.0 to 99.5% by weight and most preferred 98.0 to 99.1% by weight.

In this invention, when 98.0 to 99.8% by weight of hydrogen is removed and the polymerization undergoes under these conditions of hydrogen content, very high molecular weight polymer can be achieved this way and Charpy impact strength and Flexural modulus are improved. It was surprisingly found that working outside the range of 98.0 to 99.8% by weight of hydrogen removal, the inventive effect of obtaining very high molecular weight polymer and improving Charpy Impact an Flexural Modulus could not be observed to the same extend. The effect was more pronounced in the ranges mentioned to be preferred.

The polymerization conditions of the second reactor are notably different from that of the first reactor. The temperature in the second reactor ranges from 65-90° C., preferably 68-80° C. The molar ratio of hydrogen to ethylene is not controlled in this reactor since hydrogen is not fed into the second reactor. Hydrogen in the second reactor is the hydrogen left over from the first reactor that remains in slurry stream after being flashed at the hydrogen removal unit. Polymerization pressure in the second reactor ranges from 100-3000 kPa, preferably 150-900 kPa, more preferably 150-400 kPa.

Hydrogen removal is the comparison result of the amount of the hydrogen present in the slurry mixture before and after passing through the hydrogen removal unit. The calculation of hydrogen removal is performed according to the measurement of gas composition in the first and the second reactor by gas chromatography.

After the substantial amount of hydrogen is removed to achieve the inventive concentration, slurry from the hydrogen removal unit is transferred to the second reactor to continue the polymerization. In this reactor, ethylene can be polymerized with or without α-olefin comonomer to form HMW1 polyethylene in the presence of the LMW polyethylene or MMW polyethylene obtained from the first reactor. The α-olefin comomer that is useful for the copolymerization includes C₄₋₁₂, preferably 1-butene and 1-hexene.

After the polymerization in the second reactor, the slurry obtained is transferred to the third reactor to continue the polymerization.

The HMW2 is produced in the third reactor by polymerizing ethylene with optionally α-olefin comonomer at the presence of LMW or MMW and HMW1 obtained from the first and second reactor. The α-olefin comonomer that is useful for the copolymerization include C₄₋₁₂, preferably 1-butene and 1-hexene.

In order to obtain the target Density and the target MI in the third reactor, the polymerization conditions are controlled and adjusted. However, the polymerization conditions of the third reactor are notably different from the first and second reactor. The temperature in the third reactor ranges from 68-90° C. preferably 68-80° C. Hydrogen is optionally fed to the third reactor so as to control the molecular weight of polyethylene. The molar ratio of hydrogen to ethylene can be varied depending up on the target MI. However, the preferred molar ratio ranges from 0.01-2.0. Polymerization pressure in the third reactor ranges from 250-900 kPa, preferably 250-600 kPa, and is controlled by the addition of inert gas such as nitrogen.

The amount of LMW or MMW present in the multimodal polyethylene composition of the present invention is 30-65 parts by weight. HMW1 present in the polyethylene of the present invention is 5-40 parts by weight and HMW2 present in the polyethylene of the present invention is 10-60 parts by weight. It is possible that HMW1>HMW2 or HMW1<HMW2 depending on the polymerization conditions employed.

The final (free-flow) multimodal polyethylene composition is obtained by separating hexane from the slurry discharged from the third reactor.

Definitions and Measurement Methods

MI₂, MI₅, MI₂₁ ^(a): Melt flow index (MI) of polyethylene was measured according to ASTM D 1238 and indicated in g/10 min that determines the flowability of polymer under testing condition at 190° C. with load 2.16 kg, 5 kg and 21.6 kg, respectively.

MI₅ ^(b): Melt flow rate (MI) is determined according to ISO1133 at 190° C. and is indicated in g/10 min. The load under the melt flow rate is determined and indicated as a subscript, MI₅ is measured under 5 kg load.

Density^(a): Density of polyethylene was measured by observing the level to which a pellet sinks in a liquid column gradient tube, in comparison with standards of known density. This method is determination of the solid plastic after annealing at 120° C. follow ASTM D 1505.

Density^(b): Density is measured according to ISO 1183 and showed in g/cm³ unit.

Intrinsic Viscosity (IV): The test method covers the determination of the dilute solution viscosity of HDPE at 135° C. or Ultra High Molecular weight Polyethylene (UHMWPE) at 150° C. The polymeric solution was prepared by dissolving polymer in Decalin with 0.2% wt/vol stabilizer (Irganox 1010 or equivalent). The details are given for the determination of IV followed ASTM D2515.

Flexural Modulus: The specimen was prepared and performed the test according to ISO178. The flexural tests were done using a universal testing machine equipped with three point bending fixture.

Comonomer content: Quantitative ¹³C-NMR was used to determine the comonomer content in polyethylene copolymer. The characteristic signals corresponding to the incorporation of 1-butene-1 and/or 1-hexene were identified and calculated to obtain the amount of comonomer in the polymer.

Crystallinity: The crystallinity is frequently used for characterization by Differential Scanning Calorimetry (DSC) follow ASTM D 3418. Samples were identified by peak temperature and enthalpy, as well as the % crystallinity was calculated from the peak area. Molecular weight and Polydispersity index (PDI): The weight average molecular weight (Mw), the number average molecular weight (Mn) and the Z average molecular weight (M_(z)) in g/mol were analysed by gel permeation chromatography (GPC). Polydispersity index (PDI) was calculated by Mw/Mn

Around 8 mg of sample was dissolved in 8 ml of 1,2,4-trichlorobenzene at 160° C. for 90 min. Then the sample solution, 200 μl, was injected into the high temperature GPC with IR5, an infared detector (Polymer Char, Spain) with flow rate of 0.5 ml/min at 145° C. in column zone and 160° C. in detector zone. The data was processed by GPC One® software, Polymer Char, Spain.

Stress at yield: The tensile testing was performed by elongating a specimen and measured the load carrier following the specimen dimension using Universal testing machine. A variety of tensile properties can be extracted from the stress-strain curve. The tensile stress at the yield is the point with an increase in strain does not result in an increase in stress.

Eta747: The viscosity at shear stress 747 Pa was performed on DHR-3 controlled stress rotational rheometer from TA instrument. Using parallel plate geometry of 25 mm and measurement Gap 1.8 mm. The sample was melt in Compression molding at 190° C. for 5 minutes. The creep stress 747 Pa was applied to sample under nitrogen at 190° C. The strain deformation (γ) was monitored in the function of time. Eta747 was calculated from strain deformation at equilibrium creep time.

Lamella thickness (L): The structure parameters were analyzed by synchrotron X-ray analysis in terms of Small Angle X-ray Scattering,SAXS and Wide Angle X-ray Diffraction,WAXD measurement. The energy of X-ray radiation is 9 KeV with the wavelength of 0.1540 nm. All of samples (polyethylene powder) were preheated at 170° C. for 5 minutes and then were compressed in 1.0×1.0×0.5 cm³ aluminum plate at same temperature under pressure at 30 MPa for 4 minutes. After that, all samples are removed immediately at room temperature.

The lamellar repeating period were determined by using the plot of Lorentz corrected scattering intensities as a function of scattering vector as follows:

$L = \frac{2\pi}{q_{m}}$

Where L is the lamellar repeating period and q_(m) is the maximum peak position of scattering vector.

Other structure parameters including crystalline (Lc) and amorphous thickness (La) were calculated by means of the normalized one-dimensional (1D) correlation function. The equation is below:

${K(z)} = {{\langle{\left\lbrack {{\eta \left( z^{\prime} \right)} - {\langle\eta\rangle}} \right\rbrack \left\lbrack {{\eta \left( {z + z^{\prime}} \right)} - {\langle\eta\rangle}} \right\rbrack}\rangle} = {2{\underset{0}{\int\limits^{\infty}}{(\pi)^{- 1}q^{2}{I(q)}{\cos ({qz})}{dq}}}}}$

where < ) designated as the ensemble average. η(z) and <η) are the electron density along the lamellar normal and the averaged electron density, respectively. q is scattering vector and I(q) is intensity at q position. Finally, Lc is estimated from the plot between K(z) and z as shown in FIGURE below. The amorphous thickness can be evaluated by using this equation: L=Le+La.

Dynamic mechanical analysis (DMA): DMA measurements were performed in tensile mode using TA instrument Q800. The sample were preheated at 170° C. for 5 minutes and then compressed under pressure (30 bar) at 170° C. for 4 minutes. The sample was cut in the rectangular shape of 0.65×2×0.05 cm3. The sample was loaded with force track at 125% and the strain was modulated with amplitude of 0.03% at frequency of 40 Hz. The temperature was changed continuously at a heating rate of 1° C./min from −70° C. to 120° C. The result obtained from DMA is an α-relaxation process temperature.

Strain hardening modulus <Gp>: Strain hardening modulus can be determined according to ISO 18488:2015(E). In brief, compressed sheet was prepared from compounded pellet by using compression molding at 180° C. The compressed sheet thickness shall be in the range of 0.3+0.05/−0.03 mm. After pressing, the samples are annealed at 120±2° C. for 1 hour. Then, slowly cooled down the sample to room temperature with cooling rate of less than 2° C./min. The test specimens are punched from the pressed sheets following the ISO 1872-2. The measurement is in principle a standard tensile test with 200N load cell. Prior to the testing, the specimen are kept at 80° C. for 30 min to allow thermal equilibrium. The test specimen is extended at constant speed 20±2 mm/min until the true strain reaches 12.

The strain hardening modulus <Gp> is calculated as the average difference quotient:

The average runs over all N difference quotients:

$< {Gp}>={\frac{1}{N}{\sum\limits_{i = 1}^{N}\frac{\sigma_{i + 1} - \sigma_{i}}{\lambda_{i + 1} - \lambda_{i}}}}$

The calculation of <Gp> is performed typically between draw ratio 8 and 12 and expressed in unit MPa.

Accelerated Creep Test (ACT): The test is performed by HESSEL Ingenieurtechnik GmbH following the PAS 1075 standard and expressed the result time in hour (h).

Charpy impact strength: Charpy impact strength is determined according to ISO 179 at 23° C., 0° C. and −30° C. and showed in the unit kJ/m².

Pressure resistance: Pressure resistance was evaluated on 32 mm SDR 11 pipes produced by CINCINNATI extruders according to ISO 1167. The time to failure is determined in hours.

EXPERIMENTAL AND EXAMPLES Examples Regarding the Multimodal Polyethylene Composition

The medium or high density polyethylene preparation was carried out in three reactors in series. Ethylene, hydrogen, hexane, catalyst and TEA (triethyl aluminum) co-catalyst were fed into a first reactor in the amounts shown in Table 1. A commercial available Ziegler-Natta catalyst was used. The catalyst preparation is for example described in Hungary patent application 08 00771r. The polymerization in first reactor was carried out to make a low molecular weight polyethylene or medium molecular weight polyethylene. All of polymerized slurry polymer from first reactor was then transferred to a hydrogen removal unit to remove unreacted gas and some of hexane from polymer. The operating pressure in the hydrogen removal unit was be varied in a range of 100 to 115 kPa (abs) where residual hydrogen was removed more than 98% by weight but not more than 99.8% by weight from hexane before transferring to a second polymerization reactor. Some fresh hexane, ethylene and/or comonomer were fed into second reactor to produce first high molecular weight polyethylene (HMW1). All of polymerized polymer from second reactor was fed into the third reactor which produce second high molecular weight polyethylene (HMW2). Ethylene, comonomer, hexane and/or hydrogen were fed into the third reactor.

Comparative Example 1 (CE1)

A homopolymer was produced in first reactor to obtain a low molecular weight portion before transferring such polymer to hydrogen removal unit. Reactant mixture was introduced into the hydrogen removal unit to separate the unreacted mixture from the polymer. Residual hydrogen was removed 97.6% by weight when hydrogen removal unit was operated at pressure of 150 kPa (abs). The low molecular weight polymer was then transferred to the second reactor to produce a first high molecular weight polymer. Final, produced polymer from second reactor was transferred to the third reactor to create a second high molecular weight polymer. In third, a copolymerization was carried out by feeding 1-butene as a comonomer.

Example 1 (E1)

Example 1 was carried out in the same manner as Comparative Example 1 except that the hydrogen removal unit was operated at pressure of 115 kPa (abs). The residual of hydrogen from first reactor was removed 98.0% by weight. Characteristic properties of these multimodal polymers are shown in Table 2. As it can be seen, an improvement of stiffness-impact balance was observed when the percentage of removed hydrogen residual increased compared with the properties of Comparative Example 1.

Example 2 (E2)

Example 2 was carried out in the same manner as Comparative Example 1 except that the hydrogen removal unit was operated at pressure of 105 kPa (abs). The residual hydrogen from the first reactor was removed to an extend of 99.1% by weight. The operational of hydrogen removal unit under this pressure leads to an expansion of a polymer properties range. As seen in Table 2, a final melt flow rate of E2 was lower than a final melt flow rate of CE1 resulted in an improvement of Charpy impact strength while still maintained the flexural modulus.

Comparative Example 2 (CE2)

Comparative Example 2 was carried out in the same manner as Comparative Example 1 except that the hydrogen removal unit was operated at pressure of 102 kPa (abs). The residual of hydrogen from first reactor was removed to an extend of 99.9% by weight. The operational of hydrogen removal unit under this pressure leads to an expansion of a polymer properties range. As seen in Table 2, the final melt flow rate and a density of CE2 were quite similar to a final melt flow rate and a density of E2. A decay of Charpy impact strength was showed in CE2 compared to E2.

Comparative Example 3 (CE3)

A homopolymer was produced in a first reactor to obtain a low molecular weight portion before transferring the polymer to a hydrogen removal unit. Reactant mixture was introduced into the hydrogen removal unit to separate the unreacted mixture from the polymer. Hydrogen residual was removed to an extend of 97.9% by weight when hydrogen removal unit was operated at pressure of 150 kPa (abs). The low molecular weight polymer was then transferred to a second reactor to produce an ultra high molecular weight polymer. In the second reactor, a copolymerization was carried out by feeding 1-butene as a comonomer. Finally, in-situ bimodal copolymer from second reactor was transferred to a third reactor to create a high molecular weight copolymer portion. Characteristic properties of this multimodal polymers is shown in Table 2. A significant improvement in Charpy impact strength at room temperature could be obtained by decreasing a density of final polymer when co-polymer was produced in both the second and the third reactor.

Example 3 (E3)

Example 3 was carried out in the same manner as Comparative Example 3 except that the hydrogen removal unit was operated at pressure of 105 kPa (abs). The residual of hydrogen from first reactor was removed to an extend of 98.8% by weight. The polymer obtained by this process operation had a melt flow rate of 0.195 g/10 min (5 kg loading) lower than such value obtained from CE3. As seen in Table 2, it revealed an improvement of stiffness-impact balance when the percentage of removed hydrogen residual increases compared with the properties of Comparative Example 3.

Example 4 (E4)

A homopolymer was produced in the first reactor to obtain a medium molecular weight portion before transferring such polymer to hydrogen removal unit. The hydrogen removal unit was operated at pressure of 105 kPa (abs) to separate the unreacted mixture from the polymer. The residual of hydrogen from first reactor was removed to an extend of 98.9% by weight. The medium molecular weight polymer was then transferred to the second reactor to produce a first ultra high molecular weight polymer. Finally, produced polymer from second reactor was transferred to the third reactor to create a second ultra high molecular weight polymer. The second and third reactors are operated under hydrogen depleted polyethylene polymerization. The processable in-situ ultra high molecular weight polyethylene is produced by this process operation leads to an excellent improvement of Charpy impact strength while still maintained the flexural modulus. The conventional UHMWPE with very high IV was known that it was unable to measured MI21. The inventive example E4 with IV of 9 dl/g show good melt flow ability beyond the known art.

Comparative Example 4 (CE4)

A homopolymer was produced in first reactor to obtain a low molecular weight portion before transferring such polymer to hydrogen removal unit. Reactant mixture was introduced into the hydrogen removal unit to separate the unreacted mixture from the polymer. Residual hydrogen was removed 97.6% by weight when hydrogen removal unit was operated at pressure of 150 kPa (abs). The low molecular weight polymer was then transferred to the second reactor to produce a first high molecular weight polymer. Final, produced polymer from second reactor was transferred to the third reactor to create a second high molecular weight polymer. In third, a copolymerization was carried out by feeding 1-butene as a comonomer. As seen in Table 2 and 3, the final melt flow rate of CE4 were quite similar to a final melt flow rate of E5. A decay of charpy impact and flexural modulus were showed in CE4 compared to E5, even it showed lower density of E5.

Example 5 (E5)

Example 5 was carried out in the same manner as Comparative Example 4 except that the hydrogen removal unit was operated at pressure of 115 kPa (abs). The residual of hydrogen from first reactor was removed to an extend of 98.5% by weight. The polymer obtained by this process operation had a melt flow rate of 48 g/10 min (5 kg loading) lower than such value obtained from CE3. As seen in Table 2, it revealed an improvement of stiffness-impact balance when the percentage of removed hydrogen residual increases compared with the properties of Comparative Example 4.

Example 6 (E6)

Example 6 was carried out in the same manner as Example 4 except that the comonomer feeding in the third ultra high molecular weight polyethylene. The polymer produced by this process leads to an excellent improvement of Charpy impact strength while still maintained the flexural modulus. As shown in table 2, the inventive example 6 with IV of 23 dl/g show the high impact strength (one notched impact without break) and flexural modulus as compared to comparative samples, however, the melt flow index is unmeasurable due to high viscosity and high Mw.

TABLE 1 CE1 E1 E2 CE2 CE3 W_(A), % 55 55 55 55 45 W_(B), % 20 20 20 20 25 W_(C), % 25 25 25 25 30 First reactor Polymerization type Homo Homo Homo Homo Homo Temperature, ° C. 80 80 80 80 80 Total pressure, kPa 800 800 800 800 800 Ethylene, g 1,100.72 1,100.70 1,100.86 1,100.74 900.30 Hydrogen, g 1.62 1.62 1.55 1.55 2.97 Hydrogen removal unit Pressure, kPa (abs) 150 115 105 102 150 Hydrogen remove, % 97.6 98.0 99.1 99.9 97.9 Second reactor Polymerization type Homo Homo Homo Homo Copo Temperature, ° C. 70 70 70 70 70 Total pressure, kPa 250 250 250 250 250 Ethylene, g 400.52 400.81 400.35 400.06 500.17 Hydrogen, g 0 0 0 0 0 1-butene, g 0 0 0 0 18.84 Third reactor Polymerization type Copo Copo Copo Copo Copo Temperature, ° C. 70 70 70 70 70 Total pressure, kPa 400 400 400 400 400 Ethylene, g 500.74 500.11 500.30 500.63 600.02 Hydrogen, g 0 0.001 0.001 0.001 0 1-butene, g 35.05 30.01 30.03 30.04 60.01 E3 E4 CE4 E5 E6 W_(A), % 45 30 50 50 30 W_(B), % 25 30 10 10 30 W_(C), % 30 40 40 40 40 First reactor Polymerization type Homo Homo Homo Homo Homo Temperature, ° C. 80 80 80 80 80 Total pressure, kPa 800 800 800 800 800 Ethylene, g 900.30 540.50 725.21 725.57 485.70 Hydrogen, g 2.99 1.34 1.13 1.13 1.23 Hydrogen removal unit Pressure, kPa (abs) 105 105 150 115 105 Hydrogen remove, % 98.8 98.9 97.7 98.5 98.3 Second reactor Polymerization type Copo Homo Copo Copo Homo Temperature, ° C. 70 70 80 80 70 Total pressure, kPa 250 400 300 300 400 Ethylene, g 500.31 540.36 145.35 145.21 485.78 Hydrogen, g 0 0 0 0 0 1-butene, g 18.91 0 8 8 0 Third reactor Polymerization type Copo Homo Copo Copo Copo Temperature, ° C. 70 80 80 80 70 Total pressure, kPa 400 600 600 600 600 Ethylene, g 601.19 720.60 580.53 580.46 647.54 Hydrogen, g 0.001 0 0.59 1.37 0 1-butene, g 60.04 0 27 27 20.60 W_(A) means percent by weight of Polymer in the first reactor W_(B) means percent by weight of Polymer in the second reactor W_(C) means percent by weight of Polymer in the third reactor

TABLE 2 CE1 E1 E2 CE2 CE3 Powder MI₅, g/10 min 0.474 0.372 0.240 0.242 0.275 MI₂₁, g/10 min 13.83 10.80 7.38 7.23 6.40 Density, g/cm³ 0.9565 0.9578 0.9555 0.9567 0.9441 IV, dl/g — — — — — Mw 276,413 244,279 291,295 319,487 252,160 Mn 8,877 8,724 8,843 8,472 8,016 Mz 2,788,607 2,370,678 3,401,041 4,135,007 1,638,224 PDI 31 28 33 38 31 Pellet MI₅, g/10 min 0.436 0.410 0.232 0.199 0.298 MI₂₁, g/10 min 14.46 11.68 7.876 6.696 7.485 Density, g/cm³ 0.9577 0.9574 0.9568 0.9566 0.9442 IV, dl/g 2.97 3.03 3.52 3.64 3.12 % Crystallinity, % 64.70 67.24 64.78 66.16 57.49 Charpy, 23.5 29.9 35.3 30.5 47.9 23° C., kJ/m² Flexural 1,130 1,210. 1,123 1,123 727 modulus, MPa E3 E4 CE4 E5 E6 Powder MI₅, g/10 min 0.200 — 54.80 48.07 NA MI₂₁, g/10 min 4.81 0.145 641 653 NA Density, g/cm³ 0.9438 0.9534 0.9606 0.9590 0.9409 IV, dl/g — 9.00 1.07 1.06 23 Mw 306,468 868,813 77,334 91,752 1,269,336 Mn 7,637 24,107 5,400 6,035 23,450 Mz 2,643,953 5,112,060 667,276 1,027,956 5,262,195 PDI 40 36 14 15 54.13 Pellet MI₅, g/10 min 0.195 — 60.62 55.47 — MI₂₁, g/10 min 4.604 — 713.1 752.2 — Density, g/cm³ 0.9440 — 0.9608 0.9594 — IV, dl/g 3.37 9.00 1.0 1.1 23 % Crystallinity, % 54.05 68.23 69.52 65.64 58.20 Charpy, 50.9 84.4 1.5 1.8 85.41 23° C., kJ/m² Flexural 785 1,109 1,147 1,196 890 modulus, MPa

Pipe-Related Examples

The obtaining multimodal PE product from the third reactor was dried and the resulting powder sent to a finishing operation where it was compounded with carbon black 2-2.5 wt % in extruder at 210° C. under nitrogen atmosphere with 2000 ppm Ca/Zn stearate and 3000 ppm hindered phenol/phosphate stabilizers and, then, pelletized. Density and MI were obtained using the pelletized resins.

Plastic pipe is produced by extruding molten polymer through an annular die. The pipe is formed by passing the molten extrudate through a sizing sleeve and then to a cooling tank where water is sprayed on the outer surface. Solidification proceeds from the outer surface radially inward.

Inventive example 7 (E7), Inventive example 8 (E8) and comparative example 5 (CE5) were synthesized by multimodal technology following the process described above and 1-hexene is used as comonomer. The Comparative 5 was prepared by bimodal technology containing LMW and HMW with 1-hexene comonomer. The Comparative example, CE6, was selected from Example 7 of EP 1417260 B1.

Polymerization conditions and polymer properties are shown in Table 3-4, respectively. Testing results and analysis were applied and recorded on the compound.

TABLE 3 Polymerization condition of Inventive and Comparative examples. Examples Inventive Inventive Comparative Comparative Inventive Example Example Example Example Example Condition Unit E7(E7) E8(E8) 5 (CE5) 6(CE6) 9 (E9) 1^(st) Reactor Split ratio % 49-50 54-56 55-57 53-56 54-57 Temperature ° C. 81-85 81-85 81-85 81-85 81-85 Pressure Bar 6.0-7.0 6.0-7.0 6.0-7.0 7.0-8.0 6.0-7.0 Hexane flow rate L/h 62.5 70.6 70.4 30 65.5 Ethylene flow rate L/h 1416.1 1566.0 1566.0 1351.9 1581.4 Hydrogen flow rate NL/h 110.6 122.1 129.96 180 105.9 Catalyst flow rate g/h 6.4 6.5 6.3 1.6 6.4 2^(nd) Reactor Split ratio % 10-12 11-13 10-12 43-47 10-12 Temperature ° C. 70-75 70-75 70-75 70-75 70-75 Pressure Bar 1.5-3.0 1.5-3.0 1.5-3.0 1.5-3.0 1.5-3.0 Hexane flow rate L/h 148.6 156.7 156.6 714 146.7 Ethylene flow rate L/h 1051.0 1051.0 1051.0 2332.3 1051.0 Hydrogen flow rate NL/h 0 0 0 1.5 0 Comonomer Kg/h 0.25 0.28 0.33 0.5 0.24 Hydrogen removal % 98.92 98.77 98.82 97.2 98.85 Flash pressure Bar 0.052 0.068 0.061 0.02-0.04 0.055 Comonomer type — 1-hexene 1-hexene 1-hexene 1-butene 1-hexene 3^(rd) Reactor Split ratio % 38-40 31-33 31-33 — 31-36 Temperature ° C. 70-75 70-75 70-75 — 70-75 Pressure Bar 1.5-3.0 1.5-3.0 1.5-3.0 — 1.5-3.0 Hexane flow rate L/h 164 172 172 — 168 Ethylene flow rate L/h 2090 1760 1752 — 1965 Hydrogen flow rate NL/h 8.4 1.4 1.03 — 6.4 Comonomer Kg/h 0.91 0.6 0.72 — 0.53 Comonomer type — 1-hexene 1-hexene 1-hexene — 1-hexene

E7, E8 and E9 show higher pressure resistance at 20° C. in various hoop stress from 12.0 to 13.2 MPa than comparative examples. Even E7, E8, E9 and CE5 were produced from the inventive process, there are specific multimodal polyethylene composition to enhance superior properties in pressure resistance. The thicker of lamellar repeating period (L), amorphous thickness (La) and crystalline thickness (Lc) of E7, E8 and E9 ensure the stiffness of crystal structure resulting in the superior pressure resistance. The obtained results support the effect of ultrahigh molecular weight part with suitable 1-hexene comonomer content and distribution to polymer properties. In accordance with DMA results, the higher alpha temperature was also observed in E7, E8 and E9 than CE6. The higher alpha temperature means the stronger in crystalline portion of polymer.

TABLE 4 Polymer properties of Inventive and Comparative examples. Examples Inventive Inventive Comparative Comparative Comparative Inventive Example Example Example Example Example Example Property Unit 7 (E7) 8 (E8) 5 (CE5) 6 (CE6) 7 (CE7) 9 (E9) 1st Reactor MFR₂ ^(a) g/10 min 118 204 224 340 NA 105 2nd Reactor MFR₂ ^(a) g/10 min 6.44 2.24 2.377 NA NA 11.35 Density^(a) g/cm³ 0.9527 0.9636 0.9587 NA NA 0.9662 3rd Reactor (Black compound) Density^(b) g/cm³ 0.9595 0.9594 0.9557 0.959 0.951 0.9651 MFR₅ ^(b) g/10 min 0.275 0.222 0.167 0.21 0.21 0.113 1-butene Content % mol — — — — 0.45 1-hexene Content % mol 0.52 0.61 0.77 0.77 — 0.23 Crystallinity % 60.75 60.45 55.68 59.72 NA 68.84 Mw g/mol 275426 254894 268019 267811 226400 300178 Mn g/mol 12323 11990 13021 10136 NA 10061 Mz g/mol 1827733 1392473 1523043 186469 NA 2074528 PDI — 22.35 21.26 20.58 26.4 15.61 29.83 Stress at yield MPa 26.11 25.35 22.64 24.21 24.25 30 Eta747 Pa · s 995 995 1348 842 NA 1975 L by X-ray nm 28.72 26.78 25.83 NA NA 28.97 Lc by X-ray nm 7.33 7.23 7.10 NA NA 8.22 La by X-ray nm 21.38 19.55 18.72 NA NA 20.75 Alpha temperature ° C. 63.93 63.46 NA 58.17 NA 64.46 <Gp> by Kiwa — 44.77 ± 0.96 58.81 ± 1.12 NA 74.21 ± 3.79 NA NA Charpy impact kJ/m² 19.8 25.1 49.9 33.69 NA 21.78 (23° C.) Charpy impact kJ/m² 13.3 16.7 35.9 25.46 NA 18.66 (0° C.) Charpy impact kJ/m² 6.59 8.07 16.1 11.37 NA 13.01 (−30° C.) Pressure resistance at 20° C. 13.2 MPa h 246 108 22 NA 81.17 292 13.0 MPa h 284 146 54 NA 86.6 and 214.93 NA 12.6 MPa h 1569 317 127 NA NA NA 12.4 MPa h >2500 1157 162 156 NA >4000 12.0 MPa h >2500 >2500 278 208 NA >4000

The features disclosed in the foregoing description and in the claims may, both separately and in any combination, be material for realizing the invention in diverse forms thereof. 

1. A reactor system for a multimodal polyethylene polymerization process, comprising: (a) a first reactor; (b) a hydrogen removal unit arranged between the first reactor and a second reactor comprising at least one vessel connected with a depressurization equipment, preferably selected from vacuum pump, compressor, blower, ejector or a combination thereof, the depressurization equipment allowing to adjust an operating pressure to a pressure in a range of 100-200 kPa (abs); (c) the second reactor; and (d) a third reactor.
 2. The reactor system according to claim 1, wherein the depressurization equipment allows to adjust the operating pressure in the hydrogen removal unit to a pressure in the range of 103-145 kPa (abs), preferably 104-130 kPa (abs), most preferably 105 to 115 kPa (abs).
 3. The reactor system according to claim 1, wherein the hydrogen removal unit further contains a stripping column for the separation of hydrogen and a liquid diluent.
 4. A process for producing a multimodal polyethylene composition in the reactor system according to claim 1, comprising; (a) polymerizing ethylene in an inert hydrocarbon medium in the first reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metallocene, and hydrogen in an amount of 0.1-95% by mol with respect to the total gas present in the vapor phase in the first reactor to obtain a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g/mol or medium molecular weight polyethylene having a weight average molecular weight (Mw) of more than 90,000 to 150,000 g/mol wherein the low molecular weight polyethylene, respectively the medium molecular weight polyethylene, has a density at least 0.965 g/cm3, and the low molecular weight polyethylene has MI2 in the range from 10 to 1,000 g/10 min and the medium molecular weight polyethylene has MI2 in the range from 0.1 to 10 g/10 min; (b) removing in the hydrogen removal unit 98.0 to 99.8% by weight of the hydrogen comprised in a slurry mixture obtained from the first reactor at a pressure in the range of 103-145 kPa (abs) and transferring the obtained residual mixture to the second reactor; (c) polymerizing ethylene and optionally C₄ to C₁₂ α-olefin comonomer in the second reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metallocene, and in the presence of hydrogen in an amount obtained in step (b) to obtain a first high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a first ultra high molecular weight polyethylene in the form of a homopolymer or a copolymer having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol and transferring a resultant mixture to the third reactor; and (d) polymerizing ethylene, and optionally C₄ to C₁₂ α-olefin comonomer in the third reactor in the presence of a catalyst system, selected from Ziegler-Natta catalyst or metallocene, and hydrogen, wherein the amount of hydrogen in the third reactor is in a range of 0.1-70% by mol, preferably 0.1-60% by mol with respect to the total gas present in the vapor phase in the third reactor or optionally substantial absence of hydrogen to obtain a second high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a second ultra high molecular weight polyethylene homopolymer or copolymer having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol.
 5. The process according to claim 4, wherein the removing is removing of 98.0-99.8% by weight of the hydrogen, more preferable 98.0-99.5% by weight, and most preferred 98.0 to 99.1% by weight.
 6. The process according to claim 4, wherein the operation pressure in the hydrogen removal unit is in the range of 103-145 kPa (abs), more preferably 104-130 kPa (abs), and most preferred 105 to 115 kPa (abs).
 7. A multimodal polyethylene composition obtainable by a process according to claim 4, comprising; (A) 30 to 65 parts by weight, preferably 40 to 65 parts by weight, preferably 50 to 60 parts by weight of the low molecular weight polyethylene or the medium molecular weight polyethylene; (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight of the first high molecular weight polyethylene or the first ultra high molecular weight polyethylene; and (C) 25 to 40 parts by weight, preferably 30 to 38 party by weight of the second high molecular weight polyethylene copolymer or the second ultra high molecular weight polyethylene, wherein the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the multimodal polyethylene composition is from 10 to 60, preferably 10 to 40, preferably 20 to 30, wherein Mw and Mn are respectively determined by gel permeation chromatography.
 8. Multimodal polyethylene composition comprising: (A) 40 to 65 parts by weight, preferably 50 to 60 parts by weight of a low molecular weight polyethylene having a weight average molecular weight (Mw) of 20,000 to 90,000 g/mol or a medium molecular weight polyethylene having a weight average molecular weight (Mw) of more than 90,000 to 150,000 g/mol; (B) 10 to 20 parts by weight, preferably 10 to 15 parts by weight of a first high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a first ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol; and (C) 25 to 40 parts by weight, preferably 30 to 38 party by weight of a second high molecular weight polyethylene copolymer having a weight average molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol or a second ultra high molecular weight polyethylene having a weight average molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol, wherein the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn of the multimodal polyethylene composition is from 10 to 60, preferably 20 to 30, wherein Mw and Mn are respectively determined by gel permeation chromatography and has a crystallinity of at least 57%, preferably of at least 60% determined by differential scanning calorimetry.
 9. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has a crystallinity from 57 to 70%, preferably 60 to 70% determined by differential scanning calorimetry.
 10. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has a weight average molecular weight from 80,000 to 1,300,000 g/mol, preferably 200,000 to 400,000 g/mol, preferably 250,000 to 350,000 g/mol, measured by Gel Permeation Chromatography.
 11. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has a number average molecular weight from 5,000 to 30,000 g/mol, 8,000 to 20,000 g/mol, preferably 10,000 to 15,000 g/mol, measured by Gel Permeation Chromatography.
 12. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has a Z average molecular weight from 800,000 to 6,000.00 g/mol, 800,000 to 3,000,000 g/mol, preferably 1,000,000 to 2,500,000 g/mol, measured by Gel Permeation Chromatography.
 13. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has a density from 0.940 to 0.965 preferably 0.945 to 0.965 g/cm³, preferably from 0.958 to 0.963 g/cm³, according to ISO 1183 and/or a melt flow index MI₅ from 0.1 to 60 g/10 min, preferably 0.1 to 0.5 g/m10 min, preferably 0.1 to 0.4 g/10 min, preferably from 0.15 to 0.3 g/10 min.
 14. The multimodal polyethylene composition according to claim 13, wherein the multimodal polyethylene composition has a melt flow index MI₅ from 0.190 to 0.4 g/10 min, preferably 0.195 to 0.275 g/10 min.
 15. The multimodal polyethylene composition according to claim 7, wherein the multimodal polyethylene composition has an average lamella repeating period comprising a crystalline portion and an amorphous portion wherein the crystalline portion has an average thickness of at least 7.2 nm and the amorphous portion has an average thickness of at least 19 nm.
 16. The multimodal polyethylene composition according to claim 15, wherein the crystalline portion has an average thickness from 7.2 nm to 9.0 nm, preferably 7.2 nm to 8.5 nm, preferably 7.2 nm to 8.3 nm, preferably 7.23 to 8.22 nm; and/or the amorphous portion has an average thickness from 19 nm to 30 nm, preferably 19 nm to 25 nm, preferably 19 nm to 23 nm, preferably 19 nm to 22 nm, preferably 19.55 nm to 21.38 nm.
 17. The multimodal polyethylene composition according to claim 7, wherein an alpha temperature of multimodal polyethylene composition is at least 60° C., preferably of at least 63° C., more preferably of 63 to 70° C., most preferred from 63 to 65° C.
 18. The multimodal polyethylene composition according to claim 17, wherein the alpha temperature is from 63.46 to 64.46° C.
 19. Use of a pipe comprising the multimodal polyethylene composition according to claim 7 for applications under a pressure of at least 12.5 MPa. 